1. Field of the Invention
The present invention relates to a method of manufacturing a wiring substrate and, more particularly, a method of manufacturing a wiring substrate having a structure that enable conduction of both surface sides of a core substrate via through holes provided in the core substrate.
2. Description of the Related Art
In the prior art, there is the wiring substrate having the structure in which the wiring patterns formed on both surface sides of the core substrate are connected mutually via the conductors provided in the through holes in the core substrate.
As the method of manufacturing such wiring substrate, as shown in FIG. 1A, at first through holes 100x are formed in a core substrate 100 made of a glass epoxy resin, or the like, and then insides of the through holes 100x are cleaned by a desmear process using a permanganic acid, or the like. Then, as shown in FIG. 1B, a seed layer (not shown) is formed on both surfaces of the core substrate 100 and side surfaces of the through holes 100x by the electroless plating, and then a metal layer (not shown) is formed on the seed layer by the electroless plating utilizing the seed layer as a plating-power feeding layer, and thus first conductive layers 102 are obtained. Accordingly, both surface sides of the core substrate 100 can be brought into a conductive condition via the first conductive layer 102 provided in the through holes 100x. At this time, cavity is left in the through holes 100x in the core substrate 100 respectively.
Then, as shown in FIG. 1C, an insulating resin body 104 is filled in the cavities of the through holes 100x. Then, as shown in FIG. 1D, projected portions of the insulating resin bodies 104 projected from the first conductive layer 102 are polished on both surface sides of the core substrate 100 respectively. Thus, an upper surface and a lower surface of the insulating resin body 104 are planarized to get substantially coplanar surfaces with exposed surfaces of the first conductive layers 102 on the upper side and the lower side of the core substrate 100 respectively.
Then, as shown in FIG. 1E, a resultant structure in FIG. 1D is cleaned by applying the desmear process thereto, then a seed layer (not shown) is formed on the first conductive layer 102 on both surface sides of the core substrate 100 by the electroless plating respectively, and then a metal layer (not shown) is formed on the seed layer by the electroplating utilizing the seed layer as the plating-power feeding layer, and thus a second conductive layer 106 is obtained on both surface sides respectively.
Then, as shown in FIG. 1F, wiring patterns 108 each composed of the first and second conductive layers 102, 106 are formed on both surfaces by patterning the second conductive layer 106 and the first conductive layer 102. In this manner, the wiring patterns 108 on both surface sides of the core substrate 100 are connected mutually via the first conductive layers 102 in the through holes 100x in the core substrate 100.
In the above method of manufacturing the wiring substrate, the steps of the desmear process, the electroless plating, and the electroplating are required in the case of forming the first conductive layers 102 for enabling conduction of both surface sides of the core substrate mutually, in the through holes 100x respectively. As a result, such problems exist that the manufacturing steps become complicated because many manufacturing steps are needed, and the heavy burden of the chemicals preparation and the liquid waste processing is imposed.
In order to overcome such problems, instead of the process of forming the conductive layer in the through holes of the core substrate by the plating, there is the manufacturing method of inserting the metal posts into the through holes.
That is, as shown in FIG. 2A and FIG. 2B, the through holes 100x are formed in the core substrate 100, and then metal posts 110 are inserted into the through holes 100x. Then, as shown in FIG. 2C and FIG. 2D, a conductive layer 112 is formed on both surfaces of the core substrate 100 by the electroless plating and the electroplating respectively, and then wiring patterns 114 are formed by patterning the conductive layer 112 on both surfaces respectively. Accordingly, the wiring patterns 114 formed on both surfaces of the core substrate 100 respectively are connected mutually via the metal posts 110 in the through holes 100x. 
The wiring substrate having the structure in which the metal posts are inserted into the through holes provided in the core substrate is set forth in Patent Literature 1 (Patent Application Publication (KOKAI) 2002-289999), Patent Literature 2 (Patent Application Publication (KOKAI) 2003-220595), and Patent Literature 3 (Patent Application Publication (KOKAI) 2001-352166), for example.
FIG. 3 is an enlarged sectional view showing a behavior between the through hole 100x in the core substrate 100 and the metal post 110 in FIG. 2B.
As shown in FIG. 3, in the case of inserting the metal post 110 into the through hole 100x in the core substrate 100, a clearance G is ready to occur between the through hole 100x and the metal post 110. This is because it is difficult to reconcile an inner diameter of the through hole 100x with an outer diameter of the metal post 110 perfectly and also the defects such that the metal post 110 is bent partially upon inserting the metal post 110 into the through hole 100x, and the like are caused.
When the clearance occurs between the through hole 100x and the metal post 110, the plating is not partially applied owing to the influence of the clearance G in forming the wiring patterns 114 on both surface side of the core substrate 100, and thus there is such a possibility that the wiring patterns 114 are disconnected.
Also, in case the top end portions of the metal posts 110 are projected excessively from the core substrate 100, the projected portions of the metal posts 110 must be removed by the polishing or the etching to get the flat surface. In the prior art, since the metal posts 110 are not perfectly fixed to the through holes 100x, sometimes the metal posts 110 drop out at the time of polishing, or the etchant sinks into the clearance to etch unnecessarily the metal posts 110 at the time of etching.
As described above, in the method of inserting the metal posts 110 into the through holes 100x of the core substrate 100 according to the prior art, the disadvantages such as the connection failure between the metal posts 110 and the wiring patterns 114, and the like are ready to generate and thus it is extremely difficult to manufacture the highly-reliable wiring substrate at high yield.